The radial carcass reinforcements of such tires generally comprise several plies of textile cords, which are anchored in each bead to at least one bead core which generally has a single bead wire. The reinforcing elements of these reinforcements are wound around said bead wire from the inside to the outside, forming turn-ups, the respective ends of which are spaced radially from the axis of rotation of the tire. The severe conditions under which airplane tires are used are such that the life of the beads is short, particularly in the area of the turn-ups of the carcass reinforcement.
A substantial improvement in performance is obtained by the separating of the plies of the carcass reinforcement into two groups. The first group comprises the plies of the carcass reinforcement which are axially towards the inside in the region of the beads, these plies being then wound around a bead wire in each bead from the inside to the outside of the tire. The second group is formed of at least one axially outer ply in the region of the beads, which ply is generally wound around the bead wire from the outside to the inside of the tire. Such arrangements are known; for instance, in U.S. Pat. No. 4,244,414.
The endurance of the beads thus constituted may be improved by the presence within each bead of an additional reinforcement ply wound around the bead wire and thus forming an axially outer strand and an axially inner strand, said reinforcement ply being the ply closest to the filling or infill rubber profiled element, which is generally triangular and radially above the anchoring bead wire. An architecture of this type is disclosed in U.S. Pat. No. 5,285,835.
Likewise a solution was proposed consisting of arranging the edge(s) of said axially outer ply (plies) between the upturns of the axially inner plies.
The endurance of the beads of aircraft tires is however in need of improvement, in particular when said beads are subjected to great overloads which may impart thereto loadings causing deflections of the order of 50% of their height and more. The progress to be expected of development of the architecture of the beads would seem, at least at present, to be fairly limited, owing to the very fact of the necessary number of carcass plies, generally formed of reinforcement elements of aliphatic polyamide, in order to withstand the tension due to what is called the test pressure which, as is known, must be equal to four times the operating pressure. The large number of said carcass plies obviously involves the multiplication of the free ends of reinforcement elements, the multiplication of the interfaces between plies, greater hysteresis losses and therefore higher operating temperatures, all of which are factors favorable to increasing the fatigue of the beads and limiting their endurance.
A proposed solution is to improve the endurance of the beads of an aircraft tire lies in the replacement of the carcass plies of aliphatic polyamide by carcass plies formed of composite reinforcement elements, that is to say ones formed of yarns of different moduli as is disclosed in US patent Application US 2004/0206439.
In accordance with the aircraft tire of US 2004/0206439, inflated to high pressure, having a tread, a crown reinforcement and a radial carcass reinforcement, this radial carcass reinforcement comprised a plurality of textile reinforcement elements oriented substantially radially (that is to say forming an angle of between 80.degree. and 100.degree. with the circumferential direction), this tire was characterized in that the radial reinforcement elements of the carcass reinforcement are composite cables formed by plying at least one yarn having a modulus of elasticity in tension at least equal to 2000 cN/tex, with at least one yarn having a modulus of elasticity upon traction at most equal to 1500 cN/tex, said elasticity moduli of said yarns being measured for a tensile force equal to ten percent (10%) of the breaking load of each yarn in question. Similarly the crown reinforcement may use these cables.
Pneumatic aircraft tires are a composite of at least two primary materials: elastomer and fibers. The materials are combined to produce rubberized fibers used as reinforcement in the tire. The most common fibers in tires generally are polyester, rayon and nylon and aircraft tires also use aromatic polyamide or aramid all of which are formed into cords prior to being incorporated into elastomers. The fiber cords give a tire its shape, size, stability, load carrying capacity, fatigue and bruise resistance, to name a few characteristics.
Fiber cords are used in all the different areas of the tire where reinforcement means are required: in the carcass as a reinforcing ply for the entire carcass or in sidewall regions; in the belt or breaker structures as primary reinforcing plies or as overlays or underlays; in the bead region as flipper or chipper plies. In the different areas of the tire, the fiber cord is relied upon to provide properties specific to that region of the tire. Thus, for each area of the tire, a single type of fiber may be treated or corded in numerous ways to provide different benefits.
Prior to being incorporated into elastomer, the fiber cord is adhesively treated to ensure bonding of the fiber to the elastomer. The selected adhesive is determined so as to be compatible with the fiber being used and to permit the fiber to remain bonded to the elastomer during curing and use of the tire. An adhesive selected for use with nylon fibers will not be compatible with polyester fibers due to the different chemical structure of the adhesive and the fiber.
In treating the fiber, there are three main variables to consider: time, temperature, and tension. Each of these variables is optimized depending upon the type of fiber cord being treated, i.e. nylon versus rayon versus aramid, and the adhesive being used to create bonding between the elastomer and fiber. The time must be sufficient to allow the adhesive to bond with the fiber and set; the temperature must be sufficient to activate the adhesive; and the tension must be sufficient to ensure penetration of the adhesive, permit the fiber to move through the processing unit, and develop the requisite physical properties such as modulus, shrinkage and extensibility that are required.
In selecting a fiber cord for reinforcing a tire, the cord properties are selected to achieve desired goals. When different properties are desired and a single fiber type cannot provide the desired characteristics to the tire, different materials may be combined. A reinforcement ply may use alternating types of parallel cords.
Core/sheath types of filaments are also known. In a conventional core/sheath type of filament, such as that disclosed by U.S. Pat. No. 5,221,384 (Takahashi), the sheath is a polyamide sheath and a polyester core, with a sheath/core cross-sectional ratio of 90:10 to 10:90 down to 70:30 to 30:70. In such a cord, one skilled in the art recognizes that a true core/sheath filament exists by the resultant properties of the filament. For example, if the Takahashi filament is 10% sheath of polyamide and 90% core of polyester, the resulting properties typically follow the rules of a mixture whereby the 10% of one property of the polyamide is added to 90% of the property of the polyester. A core/sheath filament is formed through high speed spinning wherein the two different materials are spun through nested openings in the spinneret and taking advantage of die swell for the two different materials to contact and bond during orientation of the filament.
The invention of US 2005/10133137 was directed to a blended fiber cord used for reinforcing tires and pneumatic tires comprising such cords. The cords, in combination with a preferred adhesive, achieved a high degree of thermal stability allowing for the use of such cords in various reinforcing plies for a pneumatic tire and for preferred use for such cords in areas of the tire subjected to high temperatures either in curing or performance.
The object of the present invention is to achieve superior cord extensibilities while meeting the required strength performance for the severe load and deflection requirements of radial aircraft tire designs.
Unlike the tire of the US publication 2005/0133137 which employed a blended aramid and polyester cord intended primarily for use in a passenger runflat tire or agricultural tire, the present invention employs a unique merged cord employing aramid or aromatic polyamide fibers and nylon or aliphatic polyamide fibers in an aircraft tire as described below.